从 string/boost::any 映射构建 boost::options

Question

我有一张代表配置的地图。它是 std::string 和 boost::any 的映射。

该地图在开始时初始化，我希望用户能够在命令行上覆盖这些选项。

我想做的是使用 options_description::add_option() 方法从此映射构建程序选项。然而，它需要一个模板参数po::value<>，而我只有boost::any。

到目前为止，我只有代码的外壳。 m_Config 代表我的配置类，getTuples() 返回一个 std::map。 TuplePair 是 std::pair 的 typedef，并且 Tuple 包含我感兴趣的 boost::any 有没有

    po::options_description desc;
    std::for_each(m_Config.getTuples().begin(),
                  m_Config.getTuples().end(),
                  [&desc](const TuplePair& _pair)
    {
            // what goes here? :)
            // desc.add_options() ( _pair.first, po::value<???>, "");
    });

办法这样构建它，还是我需要自己做？

提前致谢！

Answer 1

boost::any 不适用于您的问题。它执行最基本形式的类型擦除：存储和（类型安全）检索，仅此而已。正如您所看到的，无法执行其他操作。正如 jhasse 指出的那样，您可以只测试您想要支持的每种类型，但这是维护的噩梦。

更好的方法是扩展 boost::any 使用的想法。不幸的是，这需要一些样板代码。如果您想尝试一下，现在邮件列表上正在讨论一个新的 Boost 库（标题为“[boost] RFC：类型擦除”），它本质上是一个通用类型擦除实用程序：您可以定义要执行的操作就像您要支持的擦除类型一样，它会生成正确的实用程序类型。 （它可以模拟 boost::any，例如，通过要求擦除的类型是可复制构造和类型安全的，并且可以模拟 boost::function<> 。

除此之外，您最好的选择可能是自己编写这样的类型 我会为您做的：

#include <boost/program_options.hpp>
#include <typeinfo>
#include <stdexcept>

namespace po = boost::program_options;

class any_option
{
public: 
    any_option() :
    mContent(0) // no content
    {}

    template <typename T>
    any_option(const T& value) :
    mContent(new holder<T>(value))
    {
        // above is where the erasure happens,
        // holder<T> inherits from our non-template
        // base class, which will make virtual calls
        // to the actual implementation; see below
    }

    any_option(const any_option& other) :
    mContent(other.empty() ? 0 : other.mContent->clone())
    {
        // note we need an explicit clone method to copy,
        // since with an erased type it's impossible
    }

    any_option& operator=(any_option other)
    {
        // copy-and-swap idiom is short and sweet
        swap(*this, other);

        return *this;
    }

    ~any_option()
    {
        // delete our content when we're done
        delete mContent;
    }

    bool empty() const
    {
        return !mContent;
    }

    friend void swap(any_option& first, any_option& second)
    {
        std::swap(first.mContent, second.mContent);
    }

    // now we define the interface we'd like to support through erasure:

    // getting the data out if we know the type will be useful,
    // just like boost::any. (defined as friend free-function)
    template <typename T>
    friend T* any_option_cast(any_option*);

    // and the ability to query the type
    const std::type_info& type() const
    {
        return mContent->type(); // call actual function
    }

    // we also want to be able to call options_description::add_option(),
    // so we add a function that will do so (through a virtual call)
    void add_option(po::options_description desc, const char* name)
    {
        mContent->add_option(desc, name); // call actual function
    }

private:
    // done with the interface, now we define the non-template base class,
    // which has virtual functions where we need type-erased functionality
    class placeholder
    {
    public:
        virtual ~placeholder()
        {
            // allow deletion through base with virtual destructor
        }

        // the interface needed to support any_option operations:

        // need to be able to clone the stored value
        virtual placeholder* clone() const = 0;

        // need to be able to test the stored type, for safe casts
        virtual const std::type_info& type() const = 0;

        // and need to be able to perform add_option with type info
        virtual void add_option(po::options_description desc,
                                    const char* name) = 0;
    };

    // and the template derived class, which will support the interface
    template <typename T>
    class holder : public placeholder
    {
    public:
        holder(const T& value) :
        mValue(value)
        {}

        // implement the required interface:
        placeholder* clone() const
        {
            return new holder<T>(mValue);
        }

        const std::type_info& type() const
        {
            return typeid(mValue);
        }

        void add_option(po::options_description desc, const char* name)
        {
            desc.add_options()(name, po::value<T>(), "");
        }

        // finally, we have a direct value accessor
        T& value()
        {
            return mValue;
        }

    private:
        T mValue;

        // noncopyable, use cloning interface
        holder(const holder&);
        holder& operator=(const holder&);
    };

    // finally, we store a pointer to the base class
    placeholder* mContent;
};

class bad_any_option_cast :
    public std::bad_cast
{
public:
    const char* what() const throw()
    {
        return "bad_any_option_cast: failed conversion";
    }
};

template <typename T>
T* any_option_cast(any_option* anyOption)
{
    typedef any_option::holder<T> holder;

    return anyOption.type() == typeid(T) ? 
            &static_cast<holder*>(anyOption.mContent)->value() : 0; 
}

template <typename T>
const T* any_option_cast(const any_option* anyOption)
{
    // none of the operations in non-const any_option_cast
    // are mutating, so this is safe and simple (constness
    // is restored to the return value automatically)
    return any_option_cast<T>(const_cast<any_option*>(anyOption));
}

template <typename T>
T& any_option_cast(any_option& anyOption)
{
    T* result = any_option_cast(&anyOption);
    if (!result)
        throw bad_any_option_cast();

    return *result;
}

template <typename T>
const T& any_option_cast(const any_option& anyOption)
{
    return any_option_cast<T>(const_cast<any_option&>(anyOption));
}

// NOTE: My casting operator has slightly different use than
// that of boost::any. Namely, it automatically returns a reference
// to the stored value, so you don't need to (and cannot) specify it.
// If you liked the old way, feel free to peek into their source.

#include <boost/foreach.hpp>
#include <map>

int main()
{
    // (it's a good exercise to step through this with
    //  a debugger to see how it all comes together)
    typedef std::map<std::string, any_option> map_type;
    typedef map_type::value_type pair_type;

    map_type m;

    m.insert(std::make_pair("int", any_option(5)));
    m.insert(std::make_pair("double", any_option(3.14)));

    po::options_description desc;

    BOOST_FOREACH(pair_type& pair, m)
    {
        pair.second.add_option(desc, pair.first.c_str());
    }

    // etc.
}

如果有不清楚的地方请告诉我。 :)

Answer 2

template<class T>
bool any_is(const boost::any& a)
{
    try
    {
        boost::any_cast<const T&>(a);
        return true;
    }
    catch(boost::bad_any_cast&)
    {
        return false;
    }
}

// ...

    po::options_description desc;
    std::for_each(m_Config.getTuples().begin(),
                  m_Config.getTuples().end(),
                  [&desc](const TuplePair& _pair)
    {
        if(any_is<int>(_pair.first))
        {
            desc.add_options() { _pair.first, po::value<int>, ""};
        }
        else if(any_is<std::string>(_pair.first))
        {
            desc.add_options() { _pair.first, po::value<std::string>, ""};
        }
        else
        {
            // ...
        }
    });

// ...

如果您有多种类型，请考虑使用类型列表。